Computers and consumer electronics are critically dependent on the memories used to store and process information. Various types of memories, including ROM, RAM, DRAM, SRAM and flash, underlie the storage and processing capabilities of computers and consumer electronics. These different forms of memory differ with respect to speed and volatility and are optimized with respect to specific tasks to provide for efficient operation. ROM, Read Only Memory, is memory that stores programs used by computers on booting (BIOS program) and for diagnostics. Data is pre-recorded on ROM and once recorded, data cannot be removed from ROM, but rather can only be read. ROM is constructed from logic hard-wired in silicon. For this reason, ROM is a highly permanent form of memory that offers a high degree of security and is not susceptible to attacks by viruses. ROM is a non-volatile form of memory, which means that ROM retains its contents when the power is turned off. Most computers have only a few kilobytes of ROM. ROM is also widely used in calculators and in peripheral devices such as laser printers. Variations of ROM include PROM (Programmable ROM), EPROM (Erasable PROM) and EEPROM (Electrically Erasable PROM). PROM is a form of ROM that is produced in an unrecorded state and is once writable and not erasable. PROM offers purchasers the ability to record programs on a ROM medium and the flexibility of changing the program as the requirements of a particular application change. EPROM a form of ROM that is erasable under action of ultraviolet light and that can be reprogrammed. EEPROM is an electrically erasable form of ROM that can be erased through software.
RAM, Random Access Memory, is the most common type of memory found in computers and other devices. RAM is the working memory of computers and is the memory utilized by programs. Data can be written, erased and re-recorded on RAM. RAM is a volatile form of memory, which means that its contents are erased when the power is turned off. DRAM (dynamic RAM) and SRAM (static RAM) are the two most common variations of RAM. DRAM is less expensive, but slower than SRAM and is characterized by a need for constant refreshing in order to retain data. The refresh requirement of DRAM is a consequence of the mechanism of data storage in a DRAM memory cell. A DRAM memory cell includes a capacitor and data is stored as charge on the capacitor. The capacitor charge is not stored permanently, however, and shows a tendency to leak to the substrate on which the DRAM cell is located or to neighboring devices on a chip. Since leakage of charge corresponds to loss of data, the charge is periodically refreshed. The refreshing requirement drives up power consumption and underlies the volatile characteristic of DRAM. Access times for DRAM are typically on the order of 60 nsec.
SRAM is a form of RAM that retains its information content without the need to refresh as long as power is maintained. SRAM is faster than DRAM, but also more expensive because an SRAM memory cell requires more transistors than DRAM (typically 4-6 transistors as opposed to one for DRAM) and requires more space on a chip. Access times for SRAM are on the order of 10 nsec and SRAM also has a much shorter cycle time (time between successive memory accesses) than DRAM because DRAM requires a pause between successive accesses due to refreshing limitations. From a performance basis, SRAM is superior to DRAM. But given its higher cost, however, it is primarily used in cache memory applications, where high speed is essential.
Flash memory is the leading non-volatile memory used in consumer electronics. Flash is a compact form of memory that is portable and conveniently interfaced with many devices. Flash is an erasable and rewritable form of memory. Flash is the memory of choice for many modern devices including cellphones (where flash is used to store the instructions needed to send and receive calls as well as to retain phone numbers), personal digital assistants (where flash is used to store addresses, calendar entries, memos etc.) and digital cameras (where flash is the type of memory used in the erasable media cards that store pictures).
Flash memory is a type of EEPROM that relies on a floating gate to store charge. The flash memory cell is similar in construction to a transistor and includes a floating gate (typically a polysilicon layer) and a tunnel oxide layer that are inserted between the oxide of the control gate and the channel region of a transistor. Data in the form of a “0” or “1” is stored by controlling the charge of the floating gate. If no charge is stored on the floating gate, current flows from source to drain when a voltage is applied to the control gate as the transistor turns on. If a charge is stored, current is inhibited and application of a gate voltage fails to turn on the transistor. The information content in a flash memory cell is thus determined through a simple read protocol involving a determination of whether an applied gate voltage turns the transistor on.
As the demands for faster, less expensive, smaller and more efficient computers and consumer electronics become more stringent, chip manufacturers and device designers have come to recognize the deficiencies of current memory technologies and have begun to search for alternative materials and devices for storing data. There is currently a great deal of interest in identifying replacements for flash memory because of the importance of flash memory for many applications and because of several shortcomings that have been identified with flash memory. Current flash memory suffers from two important drawbacks. First, the write time of flash memory is slow (on the order of a microsecond) and limits the range of applications for flash memory. While suitable for archival storage, flash is unsuitable for use as a working memory because competitive data processing requires fast writing times. Second, the lifetime of flash memory is relatively short as the reliability of data storage in flash memory diminishes after a few hundred thousand write-erase cycles.
Three new technologies directed at obtaining a faster, more reliable replacement for flash memory are currently under development. In one technology, FRAM (Ferroelectric RAM), a ferroelectric material is used to store data. An FRAM memory cell includes a capacitor containing a ferroelectric material such as PZT that records binary information based on the orientation of the ferroelectric domains of the ferroelectric material. The ferroelectric domains can be reversibly aligned in one of two directions to define two binary states that can be distinguished in a read operation based on determining a current upon application of a short voltage pulse to the capacitor. Depending on the orientation of the ferroelectric domains relative to the electric field associated with the voltage pulse, the current induced by the voltage pulse is either high or low. A second technology, MRAM (Magnetoresistive RAM), utilizes the ferromagnetic properties of atoms. MRAM is a magnetic analogue of FRAM that relies on the ferromagnetic characteristics of a ferromagnetic material to store information. A ferromagnetic material includes domains having a magnetic dipole, where the domains can be aligned and oriented under the action of an external magnetic field. As in FRAM, the orientation of aligned magnetic domains defines two binary states that are used to record information. In one device configuration, MRAM includes a magnetic tunnel junction that includes two ferromagnetic layers separated by a tunnel oxide where the relative orientation of the magnetic domains of the two ferromagnetic layers dictates that current flow across the junction. The current flow is high when the two ferromagnetic layers have parallel magnetic domains and is low when the two ferromagnetic layers have anti-parallel domains. A third technology with the potential to replace flash memory is Ovonic Unified Memory (OUM). OUM records information through the phase of a chalcogenide phase change material. Chalcogenide phase change materials can be reversibly transformed between amorphous and crystalline states where each state may correspond to a different binary state. Since the amorphous and crystalline states differ in resistance by two or more orders of magnitude, the two states are readily distinguishable.
FRAM, MRAM and OUM all address the deficiencies of conventional flash memory. All three potential flash replacement technologies offer fast writing times and essentially endless cycle life stability. All three technologies also are non-volatile and require no static power. Development work in the three flash replacement technologies is focusing on cost, deposition and manufacturing issues. Of greatest concern is the ability to integrate the technologies into existing CMOS fabrication processes. Also of concern is the development of adequate, reliable and reproducible growth methods for forming uniform thin film layers of ferroelectric, ferromagnetic or chalcogenide materials and the compatibility of these layers with conventional silicon based materials.
An additional consideration concerns the data storage capacity of potential flash replacement technologies. Current efforts in the development of FRAM, MRAM and OUM have focused on memory cells capable of storing one bit of information per memory cell or volume of active ferroelectric, ferromagnetic or chalcogenide material. Under this assumption, it is believed that development of the three replacement flash technologies will ultimately lead to data storage capacities that are comparable to those of conventional flash. In this view, it is believed that the advantageous writing speed and reliability features of FRAM, MRAM or OUM will ultimately prevail and convince industry to drop flash and adopt a superior replacement technology. Recent advances in conventional flash memory, however, have raised the entry barrier for a replacement flash technology. These advances have led to the development of flash memory that can store two or more bits of information per data cell. As a result, the cost per stored bit of information has dropped considerably in current flash technology and the performance and cost standards for a competing replacement technology have increased commensurately.
At this point in time, it is unknown whether any of the three currently identified replacement flash technologies will prove to be better than the others and whether any of them will perform well enough to displace conventional flash technology. It is clear, however, that any replacement for flash must provide a competitive storage capacity. A need exists, therefore, for a non-volatile memory technology capable of providing a data storage capacity comparable to the two or more bit per cell storage offered by today's flash technology.